Name
Description
Event code
4352-4361 Descr.
Time to reach 100 % theta
seconds
Ref. T current
x I
n
Active meas. current
x I
n
T at a given moment
%
Max. temp. rise allowed
degrees
Temp. rise at a given moment
degrees
Hot spot estimate
degrees
Hot spot max. all.
degrees
Trip delay rem.
Remaining time to trip in seconds
Used SG
Setting group 1...8 active
5.3.18 Underexcitation protection (Q<; 40)
Synchronous machines require a certain amount of excitation to stay stable. If the excitation drops too
low a synchronous machine can drop out of step. One way for the relay to sense underexcitation is by
measuring reactive power. When the generator induces capacitive power, the value of reactive power is
negative (and thus the excitation current can be low enough for the synchronous machine to drop out
of step). Underexcitation protection supervises capacitive power and picks up when the set kvar value
is exceeded.
The image below presents the two modes of underexcitation and their protection areas. The Fixed
mode is depicted on the left, the P-dependent mode on the right.
Figure. 5.3.18 - 166. Underexcitation modes.
The outputs of the function are the START, TRIP and BLOCKED signals. The underexcitation function
uses a total of eight (8) separate setting groups which can be selected from one common source.
The function can operate on instant or time-delayed mode (DT).
The operational logic consists of the following:
• input magnitude processing
A
AQ
Q-G215
-G215
Instruction manual
Version: 2.04
238